The invention relates to an adaptive optics system incorporating a holographic wavefront sensor.
A highly spherical (or flat) wavefront is required for diffraction-limited imaging, and peak-to-valley phase errors of less than a quarter of a wavelength are generally preferred. In many cases, however, imaging is degraded due to distortions of the phase of the final wavefront. For example, when viewing scenes through the atmosphere, variations in refractive index due to pockets of hot and cold air can distort a wavefront significantly, blurring images and limiting the resolution. Imperfect or misaligned optics can also result in phase distortions which will similarly degrade image quality.
Adaptive optics is a technique which can correct for these aberrations. The process involves detecting the shape of the distorted wavefront and then applying the inverse error to return the wavefront to the desired flatness. The wavefront is typically characterized using a wavefront sensor of which there are several types. Once the phase is known, a computer calculates the correction to be applied to correct the wavefront. This correction is then applied to the incoming wavefront using one of several types of adaptive optics. Generally this process is slowed due to computational loads imposed by the detection and computational methods as well as requirements of control software to take the “sensed” data and convert this into “correction” signals for the corrective optic. For this reason there are limits on the speeds that adaptive correction can achieve in real life systems. Typical adaptive optics systems operate at around 100 Hz, with the fastest ones approaching 10 kHz. In cases of extremely rapid phase changes such as those experienced in looking through airflows around airplanes, conventional adaptive optics may not be fast enough to provide acceptable correction.
In patent application Ser. No. 11/138,727 titled “Holographic Wavefront Sensor” and incorporated herein by reference we introduced a fast holographic wavefront sensor capable of characterizing wavefronts at speeds greater than 100 kHz—some 10 times faster than those currently in use. This wavefront data must then be utilized in such a way that a correction can be applied to the wavefront with little or no time delay. Even if this wavefront sensor were inserted into a conventional adaptive optics system, the data transfer and control software bottlenecks would slow down the correction process considerably. In this patent we describe a method for tailoring the sensing parameters of the holographic wavefront sensor to our adaptive optic. In essence there will be a one-to-one relationship between a particular phase component in the wavefront and an actuator such that the detection can be used to control the actuator without any intermediate computations or control software.